1. Field of the Invention
The present invention relates to a sheet feeding device configured to feed a sheet and to an image forming apparatus.
2. Description of the Related Art
Conventionally, an image forming apparatus such as a printer and a copier is provided with a sheet feeding device configured to feed sheets stacked on an intermediate plate provided in a cassette for storing the sheets, i.e., recording media such as plain sheets, OHPs, coated sheets, one by one to an image forming portion. By the way, if the sheet fed from the sheet feeding device to the image forming portion skews or is shifted in a width direction orthogonal to a feed direction, misregistration occurs in an image to be formed on the sheet.
Then, a pair of right and left side restricting plates, i.e., side edge restricting portions, is provided in the cassette in order to restrict widthwise positions of the sheets stacked on the intermediate plate. One of the pair of side restricting plates is set as a side restricting plate having a feed reference plane to feed the sheet such that a position of an image is appropriately adjusted with a position of the sheet in the image forming portion. The side restricting plate having the reference plane will be referred to as a ‘reference side restricting plate’ hereinafter. The other one of the pair of side restricting plates will be referred to as a ‘non-reference side restricting plate’ hereinafter. This pair of side restricting plates, i.e., the reference and non-reference side restricting plates, is provided to be able to approach/separate to/from each other corresponding to a size of the sheet. For example, the pair of side restricting plates can be fixed to restrict the sheets at positions corresponding to standards of sizes of the sheets such as A and B series sheets.
A distance between the pair of side restricting plates is set such that the side restricting plates are fixed at positions slightly wider than a width of the standard sheet. This setting is established by considering unevenness of sheets during manufacturing (the JIS permits an allowance of around 3 mm in each dimension for example) and that it is difficult to accurately align edges of sheets in a case where a large number of sheets is stacked.
However, if the distance between the pair of side restricting plates is widened, there is a case when a gap is generated between the sheets stacked on the intermediate plate and the side restricting plates, so that it is necessary to abut the sheets against the reference side restricting plate to align the edges of the sheets. In order for that, the non-reference side restricting plate is provided projectively with pressing members biased by springs toward the reference side restricting plate. Then, one end of the sheets are pressed by the pressing members to press the other end of the sheets to the reference side restricting plate to align the sheets in a state in which the sheets abut against the reference plane.
Here, if a force pressing the sheets by the pressing members is insufficient, it becomes unable to press the sheets against the reference plane of the reference side restricting plate, causing such problems that the sheet cannot be fed along the adequate reference position and that the sheet is fed askew. If the force of the pressing member pressing the sheet is too strong in contrary, the sheet is strongly pressed against the reference plane, causing such problems that the sheet is fed unstably due to a friction between the sheet and the reference plane and that the sheet is deformed unnecessarily.
Then, Japanese Patent Application Laid-open No. H5-105243 has disclosed a configuration in which the non-reference side restricting plate is provided, at an upper part thereof, with a pressing member that presses sheets at an upper part among sheets stacked on the intermediate plate against the reference side restricting plate by a weak force. The non-reference side restricting plate is also provided, at a lower part thereof, with a pressing member that presses sheets located at a lower part of the sheets stacked on the intermediate plate against the reference side restricting plate by a relatively strong force. This arrangement makes it possible to sequentially feed the sheets reliably from an adequate position without applying a large force from the pressing members to the sheet to be fed. It is also possible to align the lower part of the sheets on the intermediate plate reliably to the reference position by disposing the pressing member that presses the sheets by the relatively large force at the lower part of the non-reference side restricting plate.
By the way, in the sheet feeding device of the past, a vertical position around a rear end portion of the sheets changes considerably in the configuration in which the intermediate plate turns upward along with a decrease of a stacking amount of sheets. That is, while the position near an uppermost position of the rear end portion of the sheets stacked on the intermediate plate is located at an upper part of the side restricting plate when a full load of sheets is stacked, the position of the rear end portion of the sheets is located at a lower part of the side restricting plate when the intermediate plate turns upward along with the decrease of the sheets.
Due to that, when a remaining amount of the sheets is lessened, an upper part of the few remaining sheets is pressed by the large force by the pressing member disposed at the lower part of the non-reference side restricting plate. That is, it is unable to weakly press only the sheets near the upper most position regardless of the stacking amount of the sheets in the case where the vertical position around the rear end portion of the sheets changes considerably. Due to that, when the amount of stacked sheets is lessened, the sheets are pressed to the reference plane by the large pressing force of the pressing member, causing such problems that the feed of the sheet is destabilized by a friction between the sheet and the reference plane and that the sheet is deformed.